T and B cells both comprise cell surface proteins that can be utilized as markers for differentiation and identification. One such human B cell marker is the human B lymphocyte-restricted differentiation antigen Bp35, also known as “CD20”. CD20 is expressed during early pre-B cell development and remains until plasma cell differentiation. It is believed that the CD20 molecule regulates a step in the activation process that is required for cell cycle initiation and differentiation and is usually expressed at very high levels on neoplastic B cells.
CD20 is present on both normal B cells as well as malignant B cells, whose unabated proliferation can lead to B cell lymphoma. Thus, the CD20 surface antigen has the potential of serving as a candidate for targeting of B cell lymphomas with antibodies specific to the antigen. These anti-CD20 antibodies specifically bind to the CD20 cell surface antigen of both normal and malignant B cells, leading to the destruction and depletion of B cells. Chemical agents or radioactive labels having the potential to destroy the tumor can be conjugated to the anti-CD20 antibody such that the agent is specifically delivered to the neoplastic B cell.
The use of monoclonal antibodies targeting CD20 has been described (see, for example, Weiner, Semin. Oncol., 26, 43-51 (1999); Gopal and Press, J. Lab. Clin. Med., 134, 445-450 (1999); White et al., Pharm. Sci. Technol. Today, 2, 95-101 (1999)). Rituxan™ is a chimeric anti-CD20 monoclonal antibody that has been used widely both as a single agent and together with chemotherapy in patients with newly diagnosed and relapsed lymphomas (Davis et al, J. Clin. Oncol., 17, 1851-1857 (1999); Solal-Celigny et al., Blood, 94, abstract 2802 (1999); Foran et al., J. Clin. Oncol., 18, 317-324 (2000). The use of radiolabeled antibody conjugates has also been described (for example, Bexxar™; Zelenetz et al., Blood, 94, abstract 2806 (1999)).
The interaction of antibody-antigen complex with cells of the immune system results in a wide array of responses, ranging from effector functions such as antibody-dependent cytotoxicity, mast cell degranulation, and phagocytosis to immunomodulatory signals such as regulating lymphocyte proliferation and antibody secretion. All these interactions are initiated through the binding of the Fc domain of antibodies or immune complexes to specialized cell surface receptors on hematopoietic cells. It is now well established that the diversity of cellular responses triggered by antibodies and immune complexes results from the structural heterogeneity of Fc receptors (FcRs).
One group of these receptors, FcγRs, is found on most cells of the hematopoietic lineage, and mediate both high and low affinity binding to IgG (see, for example, U.S. Pat. No. 5,877,396, incorporated herein by reference). The high affinity receptor, FcγRI, binds monomeric IgG and is expressed exclusively on macrophages and neutrophils. It is capable of mediating antibody-dependent cell-mediated cytotoxicity (ADCC) and phagocytosis in response to crosslinking by antibody. The low affinity receptors for IgG, FcγRII and FcγRIII (CD16), are responsible for effector cell responses to immune complexes and represent the FcγRs primarily involved in the inflammatory response in vivo. FcγRII is widely expressed on haematopoietic cells and functions as an inhibitory receptor on B cells, while on cells of the myeloid lineage and on platelets, FcγRII triggers ADCC, phagocytosis and the release of inflammatory mediators when crosslinked by immune complexes. FcγRIII is expressed on various leucocytes including natural killer (NK) cells, macrophages, neutrophils, eosinophils, basophils and mast cells, and mediates effector responses when crosslinked by immune complexes. It is the sole FcR on NK cells, mediating all the antibody-dependent responses on those cells. Natural killer cells are a subset of spontaneously cytotoxic lymphocytes that lytically destroy tumor cells without apparent antigen specificity or restriction by histocompatibility molecules. In addition to these well-characterized effector cell pathways, FcγRIII has been found on immature thymocytes, where it has been postulated to function in early thymocyte development.
With other receptors of the immunoglobulin Fc portion (eg. FcγRI, FcγRII, FcεRI), CD16 plays an important role in mediating autoimmunity and inflammatory responses. Studies using monoclonal antibodies against CD16 have established this receptor's role in removing immune complexes from circulation and in mediating ADCC (see for example Van de Winkel et al., Immunol. Today, 14, 215-221 (1993)). The binding of IgG with CD16 elicits NK/LGL cell activation and triggers ADCC. ADCC can be halted in the presence of high levels of soluble CD16.
It has been found (see Mathiot et al., J. Clin. Immunol., 13, 41-8 (1993)) that the level of soluble CD16 was significantly decreased in patients with multiple myeloma compared with healthy volunteers. In addition a stage-dependent decrease of soluble CD16 was observed, with a highly significant difference between stage I and stages II+III myeloma patients. Therefore, measurement of soluble CD16 in serum is both a diagnostic and a prognostic marker of myeloma, which can be useful to define and guide novel immunomodulatory therapies of the disease.
It has further been found that CD16 is present in human serum and other body fluids and is elevated at sites of inflammation (see Fleit et al., Blood, 79, 2721-8 (1992)). It appears that there are at least two forms of human CD16, type A and type B. CD16-A is expressed predominantly on the surface of macrophages, natural killer cells and large granular lymphocytes (NK/LGL), whereas CD16-B is expressed predominantly on the surface of neutrophils and monocytes.
In spite of the significant roles of CD20 and CD16 in human lymphoma and in inducement of important immunological responses, animal models are lacking which co-express the human markers. Thus, a need exists for relevant animal models for disease study and pharmaceutical drug development.